These are rechargeable AA type batteries.
AA alkaline batteries last about 40 to 50 pictures.
NiMH batteries last about tripple that on a charge.
The original NiMH batteries would lose charge when sitting idle for several weeks.
The later eneloop or precharged batteries do not.
If you are going to take more than 150 pictures you will need an extra set of batteries.
NiMH batteries do not have a memory and do not have to be discharged before charging.
You can leave all types of batteries in the camera for several months.
You should remove batteries if you are going to not-use the camera for long periods of time.
Old batteries can corrode and make a mess out of your battery compartment.